1. Field of the Invention
The present invention relates to a pneumatic tire which can discharge a static electricity generated in a vehicle body and a tire, to a road surface, and a manufacturing method of the pneumatic tire.
2. Description of the Related Art
Recently, there have been proposed a pneumatic tire including a tread rubber blended with silica at a high ratio in order to reduce the rolling resistance that largely affects a fuel consumption performance. However, compared to a tread rubber blended with carbon black at a high ratio, the electric resistance of such tread rubber is high, and accordingly, static charge generated on a vehicle body or the tire is prevented from being released to the road surface. As a result, problems like radio noises tend to occur.
Accordingly, there has been developed a pneumatic tire structured such that a tread rubber constructed by a nonconductive rubber blended with a silica or the like is provided with a conductive portion constructed by a conductive rubber blended with a carbon black or the like, whereby an electrical conduction performance can be achieved. For example, in a pneumatic tire described in the Japanese Unexamined Patent Publication No. 2000-190709, a surface of a nonconductive tread rubber is provided with a thin conductive portion in which one end is connected to a wing rubber made of a conductive rubber, and another end is terminated at an inner side in a tire width direction than a contact end, thereby forming a conduction route for discharging a static electricity.
However, in the pneumatic tire as mentioned above, when a wear makes progress to such a degree that a surface of the tread rubber disappears by a thickness of the conductive portion, the conductive portion becomes short, a position of a terminal end moves close to a contact end, and a frequency at which the conductive portion comes into contact with a road surface is widely reduced. Therefore, there has been a tendency that an electrical conduction performance after the wear is lowered. On the contrary, there can be considered to employ a strategy of forming the thick conductive portion, however, in the light of enhancing an improving effect by forming the tread rubber by the nonconductive rubber, it is desirable to employ a means which can make a volume of the conductive portion as small as possible.
Further, in a pneumatic tire described in the following Japanese Unexamined Patent Publication No. 2008-13000, a conductive route for discharging a static electricity is formed by embedding a conductive portion which is spirally continuous along a tire circumferential direction in a nonconductive tread rubber, however, in the structure mentioned above, since the conductive route becomes long in distance, there has been a tendency that an electric resistance is increased. In order to well secure the electrical conduction performance, it is desirable to employ a means which can suppress the long distance of the conductive route at least in an early stage of the wear.